As semiconductor devices are scaled to smaller dimensions, generally in the sub-0.1 .mu.m region, it becomes more difficult to fabricate transistors with high drive current and small short-channel effects (i.e., reduced threshold voltage roll-off). To this end. pocket implant processes have been implemented to reduce the threshold voltage (Vt) rolloff, reduce the nominal Vt, and thus improve the nominal drive current. The pocket implant process is a process whereby a region of dopants (referred to herein as a pocket region) opposite to the type used for source/drain (and/or source/drain extension) regions is formed adjacent to the source/drain (and/or source/drain extension) regions. The lateral extent of the pocket region is typically less than the channel length of the MOSFET such that the formation of the pocket results in a laterally non-uniform dopant region from the source and/or drain and/or drain extension to the interior of the channel region. While the pocket implant may reduce short-channel effects, it may also increase the channel surface doping nearest the drain extension (and/or source extension) tip to a significant lateral extent to the interior of the channel region. This, in turn, lowers surface mobility due to dopant scattering. So, while drive current is improved by the pocket implant due to lower nominal Vt, the drive current is not as improved as it could be due to enhanced dopant scattering.
Additionally, source and/or drain extension region (referred to hereafter as "drain extension region") doping processes have been implemented to reduce the source and/or drain extension region parasitic resistance and to reduce the Vt rolloff and thus improve the nominal drive current. To achieve both low parasitic resistance and low Vt rolloff, the drain extension regions should be of sufficient junction depth to allow for low parasitic resistance but with small gate overlap of the drain extension regions.
It is desired therefore to provide for a structure with improved pocket implant process for high drive current. It is additionally desired to have a structure allowing for use of moderate drain extension implant energies for formation of drain extension regions which have low parasitic resistance with sufficiently low gate overlap of the drain extension regions.